swift234

Clang importer: hammer over inconsistencies between property/getter/setter optionality.

When an imported Objective-C property has nullability, force that
nullability onto the result of its getter and the parameter of its
setter, so that we have consisteny nullability among the three. SILGen
assumes that this is the case, so this fixes the null_resettable-based
crash in <rdar://problem/20145910>.

Swift SVN r26198